The present invention relates to a contact structure for a switching device, in particular to a contact structure for a switching device forming a butt type, in which contact chatter at the instant of contact separation or closing caused by clearances in the mechanism, is improved to be eliminated.
Generally, the contact operating members of mechanical switching devices often consist of a combination of movable contact members forming a butt-contact and some type of link mechanism such as a toggle link mechanism for operation. A contact spring is used to exert contact pressure upon closing of the contact.
However, as there are small clearances around each of the pivots which are used for connecting or interconnecting movable contact members forming a butt-contact and operating link mechanism, and as their clearances are totalized at the point of contact, i.e. the free end of the movable contact member arm, a remarkable amount of free movement is produced regardless of the contact spring force and chattering or bounce of the contacts at the instant of contact separation or closing is bound to occur. That is, imperfect contact is present at that moment even though a sufficient amount of contact force is being applied to the contact members by the contact spring upon completion of the closing. Such chattering or bounce tends to increase arcing at each occurrence of switching and can lead to serious wear and roughness on the contacting surfaces. Furthermore, when the switching device closes or interrupts a short-circuit current, the heavy electro-magnetic repulsive forces produced by the current act on the movable contact member arm and are superimposed on that chattering, so that such imperfect contact, in extreme conditions, may result in contact welding.
Conventionally, several means have been taken in order to cancel such repulsive forces. Making circular loop circuits with a flexible conductor connected to the movable contact arms, as illustrated in the Figures of the embodiment of the present application, is one of the simplest and most reliable means to overcome such problems. However even if such means are introduced to the contact structure, the repulsive movement can not be cancelled at the instant of contact separation or closing if there are clearances on the pivot since such free movement at the pivot and the structure do not work as an exact pivoting movement at such instants. Thus, the chattering or bounce will still be serious although such structures are used.
Therefore it is very important for switching devices which deal with heavy short-circuit currents to eliminate such mechanical chattering or bounce caused by the clearances in the pivots in the mechanism.
In FIGS. 7, 8 and 9 respective cross-sectional side views of various basic conventional examples of the relationship between a movable contact arm and spring arrangements are shown. In FIGS. 8A and 9A the diameters of the apertures around each pivot are shown in exaggerated form to clearly indicate on which side the aforementioned clearances appear in the arrangement of FIGS. 8 and 9 respectively FIGS. 10 and 11 are cross sections taken along lines X--X and XI--XI in FIG. 8 and FIG. 9 respectively.
As shown in FIGS. 7 to 9 of the drawings, a contact device of a conventional electric switch includes a stationary contact rod 101, a stationary contact 102, a movable contact arm 103, a movable contact 104, a movable contact support member 105 and an operating link 106 unitarily connected together by the movable contact arm 103 and a pivot 107, a stopper given reference numeral 110 for FIG. 7, 110' for FIG. 8, and 110" for FIG. 9 for the movable contact arm 103 provided on the movable contact support member 105, a spring 111 engaged with the movable contact arm 103, and a flexible conductor 112 connected with the end portion of the movable contact arm 103 The systems of supporting the spring 111 can be classified broadly into systems where the spring is provided between the movable contact support member 105 and the movable contact arm 103 as shown in FIG. 7, and a system where the spring is provided between the movable contact arm 103 and a stationary member 113 as shown in FIGS. 8 and 9.
In the system shown in FIG. 7, where the spring 111 is located between the movable contact support member 105 and the movable contact arm 103, the force of the spring 111 does not influence the operating mechanism including the operating link 106 and thus the clearance between the pivot 107 connecting the operating link 106 and the apertures of the movable contact support member 105 and operating link 106 can not be biassed by this spring 111. In this system, chattering is always present to some extent.
Furthermore, in the system shown in FIGS. 8 and 9, the movable contact 104 is provided at one side of the pivot 107 and the spring 111 is provided at the other side on the movable contact arm 103. Also the spring 111 is a pressure spring provided on the stationary member 113, usually an insulator mounting base, and operates to impart a contact pressure under closed conditions and a bias towards the direction for opening the movable contact arm 103. By providing the spring 111 in this way, the clearance between the pivot 107 connecting the operating link 106 and the aperture of the movable contact support member 105 is perfectly biassed. Therefore, chattering is naturally decreased to some extent.
The condition of the clearances around the pivots are illustrated in FIG. 8A and FIG. 9A. In these figures, 103A is the aperture of the movement contact arm 103, 105A and 105B are the apertures of the movable contact support member 105, 106A is for the operating link 106, and 108A is for the support member 108.
However, with such prior devices, even though the spring 111 is provided between the movable contact arm 103 and the stationary member 113 shown in FIGS. 8 and 9, the clearance still appears in the movable contact arm itself, and chattering can not be perfectly eliminated.
That is, with either of the contact devices shown in FIGS. 8 and 9, the stoppers 110', 110" are respectively located at the side of the spring 111 in relation to the pivot 107 of the movable contact arm 103. In FIG. 8 the stopper 110' is located at a position intermediate of the pivot 107 of the movable contact arm 103 and the spring 111, and in FIG. 9 the stopper 110" is located at the end of the movable contact arm 103 which is further away from the spring 111. In either case, a contacting force F is expressed by the following formula: EQU F=P.times.L.sub.1 /L.sub.2
where, the operating force of the spring 111 is P, the distance between the pivot 107 and the spring 111 is L.sub.1, and the distance from the pivot 107 to the center of the movable contact 104 is L.sub.2.
However, in the prior construction shown in FIG. 8 and FIG. 10, the clearance between the aperture of the movable contact arm 103 and the pivot 107 appears below the pivot 107, and the movable contact arm 103 rotates with the stopper 110' substantially at the beginning of contact until such clearance disappears, so that contact pressure F' during that time can be expressed by the following formula: EQU F'=P.times.(D-C)(C/D&lt;1)
where the operating force of the spring is P, the distance from the stopper 110' to the spring 111 is C and the distance from the spring 111 to the center of the movable contact 104 is D. From the above formula there is a disadvantage in that the instantaneous contact pressure F' is considerably lower than the desired contact pressure F.
Also, in the prior construction shown in FIG. 9 and FIG. 11, the clearance 115 between the aperture 103A of the contact arm 103 and the pivot 107 appears above the pivot 107. Thus, at the beginning of the contact, the movable contact seems to be biased toward the desired direction to cancel the clearance around the pivot 107. In this construction, the force Q for pressing the pivot 107 by the movable contact arm 103 is expressed by the following formula: EQU Q=P.times.A/B
where the operating force of the spring 111 is P, the distance between the stopper 110 and the spring 111 is A, and the distance between the stopper 110 and the pivot 107 is B.
However, even with this construction the movable contact arm takes on a considerably unstable condition since the factor A/B&lt;1 and the mechanical resistance R of the flexible conductor 112, which is used for connection with a stationary part of the switch tend to bias the end portion of the movable contact arm upwardly as shown in FIG. 9, at or near the closed position, and when the stopper 110 is located between the spring 111 and the connected portion of flexible conductor 112, such resistance opposes the force P. This results in the closing instantaneous pressure being substantially reduced or disappearing. So, this is the disadvantage of this construction. However, with the construction previously mentioned as shown in FIGS. 1 and 8, such resistance always tend to assist the spring 111 and can be ignored.